<?php
http_response_code(404);
    include './mysql.php';
    $user_name = $_GET["user_name"];
    $sql = "select user_name from user where user_name = '{$user_name}'";
    $result = $mysqli->query($sql);
    if($result->num_rows){        
        echo "true";
        
    }else{        
        echo "false";
    }